1. Field of the Invention
The present invention relates to a well tool combining a source of elastic waves, a device which attenuates tube waves propagating along a well as a result of the emission of (seismic or acoustic) elastic waves in the well and optionally a reception set for picking up waves reflected by the discontinuities of the formations surrounding the well.
2. Description of the Prior Art
There are many well-known applications notably in low or high resolution seismic prospecting, sonic logging, etc, where the formations surrounding a well are explored by emitting waves by means of a well source and by recording the waves received by receivers placed in the same well for example, in another well or at the ground surface.
It is also well-known that emission of useful waves is accompanied by the formation of guided waves known as tube waves which propagate along the well. These guided waves are inconvenient notably because they mask certain signals reflected by the discontinuities of the surrounding formations.
A well-known process for attenuating the effects of guided waves, as described in U.S. Pat. No. 4,858,718 uses well seismic equipment comprising an impulsive source lowered in a well, associated with an attenuation means. It consists of an inflatable bladder placed above the source and connected to a local compressed gas tank or to a surface tank by means of a supply line.
French Patent FR-2,714,486 filed by the applicant notably describes another method allowing great reduction of tube waves which propagate along a well containing a liquid and which are responsible for the parasitic signals picked up by seismic receivers coupled with an underground formation crossed by the well. This method mainly consists in performing seismic prospecting operations after dispersion in the liquid, in the upper part of the well, of a substantially liquid-insoluble gas sufficiently compressible to absorb at least part of the energy carried by the tube waves towards the seismic receivers. According to an embodiment, the gas is obtained by means of a chemical reaction set off at the surface and conveyed to the desired depth by means of a tube. The constituents of the chemical reaction can also be conveyed to the desired depth separately and combined to produce the gas. The latter can also be obtained by exploding, at the required depth, an explosive charge notably based on nitromethane for example.